The Proterozoic Eon (2500 to 543 million years ago) was characterized by moderate oxic conditions at the ocean surface and sulfidic conditions at depth. This redox stratification likely restricted the availability of biologically important trace metals, affecting nitrogen cycling and primary productivity. The deep oceans became sulfidic after ~1800 million years ago, persisting for over 1000 million years. Evidence from sulfur isotope ratios and sedimentary pyrite δ34S supports this scenario. Mid-Proterozoic oceans were also nutrient-limited, with lower primary productivity and reduced availability of essential metals like iron (Fe) and molybdenum (Mo). These conditions may have constrained the evolution of eukaryotic algae, favoring coastal habitats with access to riverine metal sources. Enhanced weathering around 1250 million years ago may have alleviated nutrient limitations, facilitating eukaryotic diversification in the Neoproterozoic. The hypothesis that Proterozoic ocean chemistry influenced early eukaryote evolution provides a new perspective on the coevolution of life and environment. Further research is needed to refine these insights and understand the broader implications for Earth's history.The Proterozoic Eon (2500 to 543 million years ago) was characterized by moderate oxic conditions at the ocean surface and sulfidic conditions at depth. This redox stratification likely restricted the availability of biologically important trace metals, affecting nitrogen cycling and primary productivity. The deep oceans became sulfidic after ~1800 million years ago, persisting for over 1000 million years. Evidence from sulfur isotope ratios and sedimentary pyrite δ34S supports this scenario. Mid-Proterozoic oceans were also nutrient-limited, with lower primary productivity and reduced availability of essential metals like iron (Fe) and molybdenum (Mo). These conditions may have constrained the evolution of eukaryotic algae, favoring coastal habitats with access to riverine metal sources. Enhanced weathering around 1250 million years ago may have alleviated nutrient limitations, facilitating eukaryotic diversification in the Neoproterozoic. The hypothesis that Proterozoic ocean chemistry influenced early eukaryote evolution provides a new perspective on the coevolution of life and environment. Further research is needed to refine these insights and understand the broader implications for Earth's history.